Fiber optic, current sensors, based on the Faraday effect, have a number of advantages for remotely measuring large electrical currents. These include wide dynamic range, fast response, immunity to electromagnetic interference, small size, and low cost. Consequently, a variety of fiber optic, current sensors have been investigated in recent years. Mainly, they have employed a single mode optical fiber (SMF) of clad silica.
These sensors have not yet reached the stage of practical field use due to lack of accuracy and stability. This is mainly due to intrinsic and induced, linear birefringences that distort the Faraday rotation being measured. A particular problem arises from the inability of silica fibers to measure accurately large currents, such as surge or fault currents. Such currents are exceptionally large, as much as 180 kA under some circumstances. They generally occur due to some failure, such as a short circuit.
The Faraday effect is a phenomenon by which a linear, polarized light will rotate when propagating through a transparent material that is placed in a magnetic field in parallel to the magnetic field. The size of the rotation angle (.theta.), given in degrees, is defined as EQU .theta.=VHL (1)
where H is the strength of the magnetic field (A/m), V is the Verdet constant of the material, and L is the path length over which the magnetic field acts (m). PA1 where I is the current, .mu..sub.o is permittivity of free space, and a is the radial distance of the magnetic field from the conductor. The magnetic field is related to the magnetic induction by the simple relation: EQU B=.mu..sub.o H. (3) PA1 where .theta. is in degrees, V is the Verdet constant, and I is in kiloamperes (kA). Thus, the sensitivity of a method for measuring the current depends on how accurately the angular rotation can be measured.
The magnetic field strength is measured in terms of Amperes (A) times turns (T) per unit length (AT/m) where m is meters). Since values are expressed in terms of one turn, this factor is usually implicit, rather than explicit. Hence, the strength is customarily given in amperes (A) or kiloamperes (kA) per unit path length in meters (m).
The Verdet constant, V, is the angle of rotation divided by the magnetic field strength per unit length. The angle may be expressed in any of the customary units for angle measurement, but degrees are used here. Verdet constant values, unless otherwise indicated, are given in terms of degrees divided by field strength expressed as (kA.times.T/m)m.
The magnitude of the magnetic induction (B) around an infinite straight conductor is given by the expression: EQU B=(.mu..sub.o /4.pi.)(2I/a) (2)
Combining equations 1 through 3 gives a proportional relation between the rotation and the current such that: EQU .theta.=VI (4)
The degree of sensitivity in measuring the angular rotation is influenced by another factor; birefringence. Birefringence arises primarily from stresses that result from bending, or otherwise distorting, a fiber in its disposition. The sources of linear birefringence in single mode fibers include residual stress from fabrication, bending, contact, and thermal stresses (Yamashita et al., "Extremely Small Stress-optic Coefficient Glass Single Mode Fibers For Current Sensor", Optical Fiber Sensors, Sapporo Japan, paper We2-4, page 168 (1996) ("Yamashita").
The stress-induced birefringence is quantified in terms of a coefficient, called the photoelastic constant (or the photoelastic coefficient). The photoelastic coefficient (B.sub.p) may be defined as the coefficient relating the difference in the refractive indices in the stress direction (n(par)) and in the pependicular direction (n(per)), to the magnitude of the applied stress: EQU n(par)-n(per)=B.sub.p.sigma. (5)
It may also be regarded as the phase shift measured in units of wavelength in nanometers (nm) per path length in centimeters (cm) divided by the stress in kilograms per square centimeter (kg/cma.sup.2). The values then are in units of (nm/cm divided by kg/cm.sup.2).
An ideal glass fiber would have a photoelastic coefficient of zero, thereby nullifying any effect of stress-induced birefringence. However, this has proven difficult to obtain in conjunction with other desired properties.
Therefore, a near-zero value, e.g., a value within a range of -0.2 to 0.2, has been considered adequate for some purposes.
In measuring a surge current, it is important to keep the angle of rotation below 90 degrees. With glass fibers having large Verdet constants, a fault current measurement is apt to create an angle of rotation greater than 90 degrees. The angle of rotation greater than 90 degrees will register the same as an angle of less than 90 degrees. In contrast, a device having a glass fiber with a low Verdet constant will not have as great an angle of rotation when measuring a large fault current. Therefore, it will accurately measure such currents.
It is a purpose of the present invention to provide an improved method and device for measuring large currents, such as surge and fault currents.
Another purpose is to provide a glass that is adapted to use in such improved method and device.
A further purpose is to provide a method of producing a glass having a near-zero photoelastic coefficient in conjunction with a low Verdet constant.
A still further purpose is to provide a method of reducing the photoelastic coefficient of a glass having a low Verdet constant.